The invention relates to a method for the mechanical disintegration of biogenic sewage sludge, for which sewage sludge flows from a first connecting region to a treatment device, which works under rotation and stresses the sewage sludge with severing parts, and from the treatment device to a second connecting region.
The sewage sludge under discussion here is at a biological purification stage, at which organic components, which can be decomposed easily, are converted into inorganic end products such as water, carbon dioxide or nitrogen. In order to accelerate this process, microorganisms are mixed with the sewage sludge, sludge flakes or aggregates of very fine suspended matter and microorganisms being formed. These aggregates comprise cells, formed by microorganisms. The effectiveness of the biological cleaning stage is improved by disintegrating the sewage sludge. The disintegration of the sewage sludge is understood to be the destruction of aggregates and cells.
A known method of disintegration a sewage sludge includes passing the sewage sludge through an apparatus including a stirrer shaft with a plurality of stirring discs mounted on the shaft in a spaced relationship to each other. The space between the stirring discs is filled with grinding bodies, e.g., balls which destroy the sludge. At the outlet of the apparatus, there is provided a screen for preventing the grinding bodies from being carried with the sludge. The above-discussed apparatus does not permit to achieve a high degree of the disintegration of aggregates and cells containing in the sludge. Moreover the screen at the outlet of the apparatus becomes rapidly clogged, which requires its often replacement which substantially increases operational and maintenance costs.
A device for mixing is known (DE 40 29 824 A1, DE 197 19 638 A1), which is a rapidly rotating pipe-head formation, having a head with radial openings and radial wings. This pipe-head formation is used for admixing a flocculating agent to the sewage sludge. From a first connecting region, the flocculating agent passes through the pipe-head formation and leaves the head in the sewage sludge. In the case of this main use of the rapidly rotating pipe-head formation, a noticeable disintegration of the sewage sludge is not achieved.
It is an object of the invention to devise a method of the initially mentioned type, for which, with improved continuity of the operation, a decisive increase in the disintegration is achieved. The invention, accomplishing this objective, consists of the use of a very rapidly rotating pipe-head formation, having a head with radial openings and radial wings, for the mechanical disintegration of biogenic sewage sludge.
The invention thus provides a method of the initially mentioned type, for which, as sewage sludge treatment device, a rapidly rotating pipe-head formation is used, which has a head with radial openings and radial wings and is disposed with the head in the second connecting region, which is constructed as connecting space, and for which the wings and the edges of the openings stress the flow of sewage sludge, passing through the rapidly rotating pipe-head formation. The whole of the flow of sewage sludge, which is to be disintegrated, must therefore pass through the rapidly rotating pipe-head formation, that is, pass through the very rapidly moving edges of the openings and through the wings which, as severing parts, destroy aggregates and cells of the sewage sludge.
The edges of the openings exert a shearing effect. On the other hand, the wings exert an impact action and the combination of these two effects, in conjunction with the rapid rotation, bring about the improvement in the disintegration. The sewage sludge does not have to flow through screens and separating means. A rapid rotation is understood here to be one of 700 rpm or higher; as a rule, the rotation is faster than 1000 rpm, namely 2000 to 3000 rpm. With a thickened excess sludge (6% total solids), a maximum increase in the COD (=carbon content) of 430% and of the DOC (=directly available carbon) of 350% can be achieved.
Depending on the energy entered, the rpm of the rapid rotation and the achievable stresses at the sludge particles in the flow of sewage sludge, the following treatment results (treatment stages) can be represented:
1. Partial to complete comminution of the activated sludge flakes (aggregates) present, release and partial comminution of the sludge components bound adsorptively in and at the flake, increase in the proportion of fines in the particle size distribution, change in the surface charge, slight increase in the COD (dissolved).
2. Further comminution of the activated sludge flakes, release of individual microorganisms from the flake composite, shearing of exoenzymes, polysaccharides, etc. from the cell walls of the microorganisms, incipient comminution of organic sludge components up to bacteria-available orders of magnitude, increase in the COD (dissolved), measurable increase in enzyme activities, change in the surface charge, further increase in the fines proportion in the particle size distribution, rapid acceleration of the pre-acidification in the anaerobic medium.
3. Incipient deformation of cell walls up to complete destruction of cells, secretion of cell contents, distinct increase in the COD (dissolved), decrease in enzyme activity, high loading of process water with dissolved materials (re-dissolving).
It is conceivable to let the sewage sludge flow so that it enters the head and leaves the pipe-head formation at the pipe. However, it is particularly appropriate and advantageous if the sewage sludge leaves the pipe-head formation at the head. The disintegration is improved since first the shear effect and then the impact effect are exerted here.
It is also particularly appropriate and advantageous if a rapidly rotating pipe-head formation is used, for which the pipe, over radial openings, changes over into the first connecting region, which is constructed as a connecting space, and also the edges of the openings in the pipe stress the sewage sludge, passing through the rapidly rotating pipe-head formation. A shear effect is thus exerted at a second place on the sewage sludge and increases the measure of disintegration.
The combination of the edges of the openings of the head with the edges of the openings of the pipe causes the sewage sludge, which is to be disintegrated, to be sheared twice by the edges of the openings. The edges of the openings are configured as sharp as possible in the form of a knife edge and, moreover, also when only the edges of the openings of the head are provided.
In the following, preferred distinguishing features of the rapidly rotating pipe-head formation, which is used pursuant to the invention, are given, which are provided individually or in any combination with one another. The openings of the head and/or of the pipe are slot-like and elongated. In the axial direction, the wings are elongated strips. An opening and a wing alternate at the head in the direction of rotation.